1. Field of the Invention
The present invention relates to imaging lenses which form an image of an object on a solid-state image sensor such as a CCD sensor or C-MOS sensor used in a compact image pickup device and more particularly to imaging lenses composed of four lenses which are built in image pickup devices mounted in mobile terminals such as mobile phones and smart phones, which are becoming increasingly compact and thin, and PDAs (Personal Digital Assistants), game consoles and information terminals such as PCs.
2. Description of the Related Art
In recent years, the market of mobile terminals with an image pickup device has been expanding increasingly. Today, most of such mobile terminals have a camera function. Nowadays, as for the camera performance of these mobile terminals, a high pixel-density camera function comparable to a digital still camera is becoming mainstream. Furthermore, for the reasons of convenience and appearance, the demand for thinner models is growing, leading to a stronger need for smaller and thinner built-in image pickup devices. With this background, an imaging lens built in an image pickup device using a high pixel-density image sensor is expected to provide higher resolution and be more compact and thinner and also to be a fast lens (namely lens with a small F-value). At the same time, the imaging lens is strongly anticipated to provide a wide angle of view so that an image of a wide object can be taken.
Conventionally, an imaging lens composed of two or three lenses has been widely used in a mobile phone with an image pickup device because it is advantageous in terms of size and cost. However, in order to meet the trend toward higher performance, many types of imaging lens composed of four lenses have been proposed because they can provide relatively high performance. However, although the number of pixels can be increased by increasing the number of constituent lenses, it is difficult to realize an imaging lens which meets the need for compactness and thinness.
Many conventional imaging lenses composed of four lenses which have been proposed include a positive first lens, a negative second lens, a positive third lens, and a fourth lens for aberration correction which are arranged in order from the object side. In most cases, this lens configuration uses a low-dispersion material with a large Abbe number for the first lens with positive power and a high-dispersion material with a small Abbe number for the second lens with negative power in order to correct chromatic aberration. However, the existence of a lens with negative power makes it difficult to shorten the distance in the optical axis direction, posing a problem that it is difficult to meet the need for thin imaging lenses though chromatic aberration can be corrected properly.
For example, U.S. Pat. No. 7,826,149 B2 (Patent Document 1) discloses an imaging lens including, in order from the object side, a first lens with positive refractive power having a convex object-side surface, an aperture stop, a second lens with negative refractive power as a double-sided aspheric lens, a third lens with positive refractive power as a double-sided aspheric meniscus lens having a concave object-side surface or a double-sided aspheric biconvex lens, and a fourth lens with negative refractive power as a double-sided aspheric biconcave lens, in which pole-change points are on the image-side surface of the fourth lens and the maximum ratio between the distance on the optical axis from the aperture stop to the image plane and the total track length is defined. In this imaging lens, in order to shorten the total track length, the refractive power of the first lens is increased and the object-side surface of the fourth lens is concave so that the image side principal point in the optical system is away from the image plane.
JP-A-2004-341512 (Patent Document 2) discloses an imaging lens including, in order from the object side, a positive meniscus lens having a convex object-side surface as a first lens, an aperture diaphragm, a meniscus lens having a convex image-side surface as a second lens, a positive meniscus lens having a convex image-side surface as a third lens, and a negative lens as a fourth lens, wherein at least one surface of the fourth lens is aspheric and the ratio between the power of the paraxial region of the fourth lens and the power at maximum light ray height and the difference between the third lens' Abbe number and the fourth lens' Abbe number are set within appropriate ranges respectively. In this imaging lens, the first lens is a meniscus lens whose object-side surface has a strong positive power and the image side principal point of the first lens is closer to the object and the image side surface of the fourth lens is concave so that the total track length is short.